01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly with butter or cooking spray.
02 - In a large bowl, beat flour, granulated sugar, softened butter, milk, eggs, baking powder, salt, and vanilla extract until mixture forms a smooth, creamy batter. Spread batter evenly into the prepared pan.
03 - Combine melted butter, brown sugar, cinnamon, and flour in a separate bowl. Mix vigorously until a thick, spreadable paste forms.
04 - Drop spoonfuls of cinnamon mixture randomly over the surface of the cake batter. Draw a knife through the batter in circular or zigzag motions to marble the cinnamon mixture throughout.
05 - Bake for 32-38 minutes until cake is golden brown and a toothpick inserted into the center emerges clean without wet batter.
06 - While cake bakes, whisk powdered sugar, milk, and vanilla extract in a small bowl until completely smooth and pourable. Adjust milk quantity to achieve desired consistency.
07 - Let cake cool in pan for 10-15 minutes. Drizzle vanilla glaze evenly over the warm surface. Slice into squares and serve while still warm.